cretaceous anoxic–oxic changes in the moldavids (carpathians, romania)

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Cretaceous anoxic–oxic changes in the Moldavids (Carpathians, Romania)

Mihaela C. Melinte-Dobrinescu a,⁎, Relu-Dumitru Roban b

a National Institute of Marine Geology and Geo-ecology, 23-25 Dimitrie Onciul Street, RO-024053 Bucharest, Romaniab University of Bucharest, Faculty of Geology and Geophysics, 1 Nicolae Bălcescu Blvd., Bucharest, Romania

a b s t r a c ta r t i c l e i n f o

Article history:Received 14 June 2009Received in revised form 26 April 2010Accepted 12 June 2010Available online 12 June 2010

Keywords:Lower CretaceousLaminated black shalesUpper Cretaceous Oceanic Red BedsRomaniaPelagites and turbidites

This study focused on the Cretaceous black shale successions, followed by red shales that crop out at theouter regions of the Romanian Carpathians, in the Moldavids. The oldest parts of the black shale unitsdeposited in an abyssal plain during Late Valanginian–Late Barremian time; they are mainly characterized byhemipelagic and pelagic muddy siliciclastic rocks and carbonates, commonly intercalated with fine-grainedturbidites.During the sedimentation of the middle part of the black shale units in the Late Barremian–Early Albianinterval, the depth of the basin increased, as the carbonate hemipelagic sedimentation was replaced by amainly siliceous one. Only a few thin turbidite intercalations are present.The youngest part (Albian pro parte) of the black shale units is characterized by a turbiditic sedimentation,with mainly sandy sequences of middle and lower deep-water fans. We may assume that the depth of thebasin continuously decreased. The presence of authigenic glauconite in the Albian sandstones suggests apalaeoenvironmental change, linked to the occurrence of oxygenated turbidity current circulation.A significant shift in the sedimentation regime in the Eastern Carpathian Moldavids took place in the LateAlbian, when Cretaceous Oceanic Red Beds (CORB) occurred. This type of sedimentation lasted up to theConiacian. The lower part of the CORBs that contains radiolarites intercalated with variegated shales,pyroclastic tuffs and thin sandstones is interpreted as a hemipelagic and pelagic sedimentation in the abyssalplain environment, where rarely turbidites occurred. Upwards, there are mainly burrowed variegated redand green shales. The youngest parts of CORBs are characterized by increased thickness and frequency of theturbidites. While the main part of the CORB is carbonate free or has very low carbonate content, the upperpart of these strata becomes rich in marl and mudstone strata, indicating a decrease of the basin-depth.The accumulation of black shales in the Eastern Carpathians during the Late Valanginian–Late Albian intervalis linked to the existence of a small, silled basin of the Moldavian Trough, in which restricted circulation ledto the density stratification of the water column, resulting in the deposition of anoxic Lower Cretaceoussediments (i.e., the black shales). Because of the tectonic deformation that took place within the Lower–Upper Cretaceous boundary interval, the restricted circulation had changed to an open circulation regime inthe Moldavian Trough. Hence, the anoxic regime was progressively replaced by an oxic one, across theAlbian–Cenomanian boundary interval. The beginning and the end of the CORBs in the Moldavid unitsdepend thus on various palaeogeographic and palaeoenvironmental settings, and it was controlled by theregional tectonic activity.

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1. Introduction

The transition from Cretaceous anoxic to oxic environments wasobserved in many Tethyan areas (Jansa et al., 1979; Arthur andPremoli Silva, 1982; Hu et al., 2005; Jansa and Hu, 2009), including theCarpathians (Švábenická et al., 1997; Bąk, 1998; Michalík et al., 2002;Wang et al., 2005; Melinte-Dobrinescu et al., 2009; Skupien et al.,2009). In the Carpathian mountain belt, the sedimentation of theUpper Cretaceous Oceanic Red Beds (referred as CORB by Hu et al.,

2005) follows the Lower Cretaceous black shale deposition. It appearsthat the development of Cretaceous black shales and CORBs issomehow causally related (Hu et al., 2006). The anoxic–oxic changesmay reflect various Earth processes, such as palaeoceanographic shift,tectonic movements, and/or climatic fluctuations that changed thebalance between carbon sources and sinks in the world ocean (Wanget al., 2005; Hu et al., 2009).

Colour of sedimentary rocks is an important indicator ofenvironmental conditions in which they have formed. Gray andblack shale occurrence is commonly linked to increases in organicmatter content, and low oxygen content in bottom waters. Inturn, CORB indicate low organic matter content, but well-oxygenatedbottom conditions.

Sedimentary Geology 235 (2011) 79–90

⁎ Corresponding author. Fax: +40 212522594.E-mail address: [email protected] (M.C. Melinte-Dobrinescu).

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Numerous dark-coloured, commonly laminated, organic-richsediments occur within Mesozoic sequences; they are frequentlyassociated with Upper Jurassic to Lower Cretaceous organic carbonenriched claystones, clayey sandstones and limestones (Wignall,1994; Pancost et al., 2004). Such beds that are more than 1-cm-thickand contain more than 1% of total organic carbon (TOC) are describedas black shales (Pettijohn, 1957; Stow et al., 2001). In general, theterm of black shales is used for any dark-colored fine-grained organiccarbon rich sediment, in which TOC contents typically range from 1 to15%. Many black shales are hemipelagites; some, such as black chertsand organic rich limestones, are pelagites; others are fine grainedturbidites (Stow et al., 1996). Black shales are also characterizedgeochemically by high Fe2+ and S2− contents and enrichments intrace elements, such as Ba, Bi, Cr, Ni, Mo, V, Zn (Nijenhuis et al., 1999;Lipinski et al., 2003). The sedimentation of black shales is mostlyascribed to oxygen-poor bottom waters, typically accumulated indeep-ocean basin setting, but they may also occur in shallower sites(Wignall and Newton, 2001).

During the mid-Cretaceous, red oxic sediments, with low contentof preserved organic carbon, replaced the deposition of organic-richblack shales in many of the Tethyan areas (Wang et al., 2005). Thistype of sedimentation is found from low to middle palaeolatitudes upto the end of the Cretaceous (Arthur and Premoli Silva, 1982; Hu et al.,2005; Melinte and Jipa, 2005; Hu et al., 2006). Probably, CORBsedimentation is a response to a palaeoclimatic and/or palaeoceano-graphic fluctuations, but also is effected by tectonic activity andsynorogenic depositional processes (Hu et al., 2005; Wagreich andKrenmayr, 2005; Neuhuber et al., 2007; Melinte-Dobrinescu et al.,2009). The occurrence of CORB may also be linked to changes inbioproductivity, triggered bymajor palaeoceanographicmodificationsof the world oceans.

In this paper, we will discuss stratigraphy and examine geneticrelationship between Lower Cretaceous organic-rich black shalesdeposition and overlaying Upper Cretaceous oceanic red beds fromthe eastern part of the Romanian Carpathian region. We alsocomment on causes of the Cretaceous anoxic–oxic changes as seenin the Moldavid nappes of the Eastern Carpathians.

2. Geological setting

The Eastern Carpathians represent a segment (over 600 km long) ofthe Carpathian tectonic chain. Inwards (westwards), this mountainouschain is bordered by the Transylvanian Basin and the easternmost partof the Pannonian Basin, while outwards (eastwards), it is bordered bythe Moldavian and Scythian Platforms (at the E) and by the MoesianPlatform (towards SE and S). The Eastern Carpathians are mainlycomposed of Jurassic–Miocene sedimentary rocks, deposited in severalbasins, folded and overthrusted on the Miocene sediments of theCarpathian Foredeep. Imbrication and internal deformation of thenappes occurred during several periods, from the Late Cretaceous up tothe Neogene (Săndulescu, 1980, 1984; Maţenco, 1997; Csontos andVörös, 2004).

Based on the age of the main deformation and the mutual arealposition, several tectonical units were recognized in the EasternCarpathians (Săndulescu, 1984), such as (from W to E): the Pieninids,the Transylvanids, the Median Dacids, the Outer Dacids, and theMoldavids (Fig. 1). The nappes of the Eastern Carpathianswere groupedinto two main tectonic units (Săndulescu, 1975), which are the Dacids(nappes mainly involved in the Cretaceous tectonic movements) andthe Moldavids (tectonic units folded mainly during the Miocene).

All the sections investigated by us are placed in the Moldavids thatrepresent a nappe system located at the outermost part of the Eastern

Carpathians. Their sedimentary and post-tectonic covers are exclusivelycomposed of Lower Cretaceous–Cenozoic rocks (Săndulescu, 1984;Ştefănescu and Micu, 1987; Ştefănescu, 1995). The oldest Moldavids,namely the Inner Moldavids (i.e., Teleajen, Macla and Audia nappes)containonlyCretaceous sediments andare located in theW(innermost)part of this nappe system. The youngest Moldavids, described as theOuter Moldavids (i.e., Tarcău, Vrancea and the Subcarpathian nappes)contain Cretaceous–Cenozoic rocks, being placed at the E (outermost)part of the nappe system.

The organic-rich black shales are the oldest sediments exposed inthe Eastern Carpathian Moldavid nappes. The western occurrence ofthese sediments is known from the innermost part of the Moldavids(i.e., the Teleajen Nappe), where they were sedimented during theLate Valanginian–Late Hauterivian (Ion, 1975; Antonescu et al., 1978).Themost complete development of the Lower Cretaceous organic-richblack shales in the Eastern Carpathians is known from the eastern partof the Inner Moldavids (i.e., the Audia Nappe) and from the OuterMoldavids, such as Tarcău and Vrancea nappes (Filipescu et al., 1963;Săndulescu, 1984; Ştefănescu and Micu, 1987), where they belong tothe Audia Formation (Ştefănescu, 1976). The Lower Cretaceousorganic-rich black shales are overlain in all the Moldavids by UpperCretaceous Oceanic Red Beds (CORB), described under differentnames (i.e., Bota-Botiţa, Cârnu-Şiclău and Tisaru formations - Băncilă,1958; Filipescu et al., 1963), but displaying a similar lithology.

3. Materials and methods

We have studied several sections located in the Inner and OuterMoldavids, displaying a continuous transition from organic-rich blackshales to CORB.However,wehave selected topresent hereinonly three ofthemost representative sections, located at the southern andcentral partsof the Eastern Carpathians, in the Audia and the Tarcău nappes. Themostcomplete investigated section is placed in the Audia Nappe, being locatedin theCarpathianBend region (Bota River, tributary of theBuzăuValley-Ain Fig. 1). Another section from the Audia Nappe, which displays only theupper part of the black-shale succession, is situated in the central part ofthe Eastern Carpathians (in the Bistriţa Valley, at Bicaz— B in Fig. 1). Theoutermost section analyzed by us belongs to the Tarcău Nappe and it ispositioned in the S Eastern Carpathians (the Covasna Valley— C in Fig. 1).All the above-mentioned sections were logged in detail, from lithologicaland sedimentological points of view. To date the studied deposits,calcareous nannoplankton analyses were achieved. Sampling wasrealized at 25 cm intervals.

4. Results

4.1. Organic-rich black shales

In the southernmost investigated section, located in the AudiaNappe (A in Fig. 1), the Audia Formation, 190 m in thickness, displaysall three members of the black shale strata (Figs. 2 and 3), which arefrom the base to the top:

(i) The Lower Member, 80 m in thickness, that contains at its lowerpart mainly laminated black shales. The upper part is composedof decimetre-thick rhythmically alternating lithic and sublithicsandstones, siltstones anddark grey shales. Theorganic richblackshales amount up to 80% in this unit. Thin dm strata of sideritelimestones and siderites are also present. Rarely, cm up to dmlimestones (mudstones and wackestones), marls and chertscould beobserved. The sandstones and the limestones of this unitappear as fining up sequences at intervals between 30 cm and300 cm (Figs. 3 and 4).

Fig. 1. a: Geologic map of Romania (simplified after Săndulescu, 1984) with the outline of the map b; b: Tectonic map of the Eastern Carpathians (modified after Săndulescu, 1984).Sections discussed in this paper: A—Bota; B—Bicaz; C—Covasna.

80 M.C. Melinte-Dobrinescu, R.-D. Roban / Sedimentary Geology 235 (2011) 79–90


81M.C. Melinte-Dobrinescu, R.-D. Roban / Sedimentary Geology 235 (2011) 79–90


The calcareous nannofossils identified within the Lower Membershow the presence of the NK3B nannofossil subzone of Bralower et al.(1989), as well as of the NC4 calcareous nannofossil zone of Roth(1983) (Fig. 2). These nannofossil zones cover the Late Valanginian–Early Hauterivian interval in the Romanian Carpathians (Melinte andMutterlose, 2001; Barbu and Melinte-Dobrinescu, 2008). The upperpart of the Lower Member is of Late Hauterivian–Late Barremian age,as indicated by the presence of the NC5 calcareous nannofossil zone(including its subzones) of Roth (1983).

(ii) TheMiddleMemberof theAudia Formation, 70 mthick, ismainlycomposed of laminated black shales, commonly interbeddedwith black cherts, the later lithological feature distinguishing thisunit from the older Lower Member (Figs. 3 and 4). The chertsyield pinch and swell structures. Sporadically, thin cm levels ofbreccias and conglomerates with granodiorite clasts occur. Thincm beds of sandstones occur in the whole unit, with higherfrequency towards the upper part of this member (Figs. 3 and 4).

The nannofossil assemblages are scarce and poorly preserved, butcan be assigned to the NC5 (upper part), NC6, NC7 and lower part ofthe NC8 calcareous nannoplankton zones of Roth (1983) (Fig. 2).These biozones are covering the Late Barremian–Early Albian interval(Scott, 2009b).

(iii) The Upper Member of the Audia Formation is composed of40 m rhythmically alternating successions of black shales anddm up to m subquartzous sandstones, with the latter showingparallel lamination and current ripples (Figs. 3 and 4). Somelevels include is a high content of macroscopically visibleglauconitic peloids. At the base of the sandstone beds, fine-grained conglomerates could be observed. The sandstonesrepresent 50% up to 80% of the Upper Member deposits. Theidentification of the calcareous nannofossil zones and subzonesNC8B, NC9 and NC10 (lower part) of Roth (1983) indicates anEarly to Late Albian age (Scott, 2009a,b) for the Upper Member(Fig. 2).

The northern studied section of the Audia Nappe is located in thecentral part of the Eastern Carpathians, at the Bicaz, Bistriţa Valley(section B in Fig. 1). There, only the Upper Member of the AudiaFormation, 40 m in thickness, is exposed (Figs. 3 and 4). It iscomprised by dm up to m thick subquartzous sandstones, locallywith fine-grained conglomerates at the base, and thin cm black shales.The sandstones constitute up to 50% of the unit (Fig. 5).

The sandstones and shales are organized in fining up sequences, of20–50 cm thick. These sequences contain sandy levels that show atthe base normal grading, than horizontal parallel- and crossed-laminated structures, followed by siltstones and shales. Based on theidentified calcareous nannoplankton assemblages, that were assignedto the NC8B, NC9A, NC9B and NC10 zones, the age of the studiedsuccession is Early to Late Albian.

We also investigated the Lower Cretaceous black shales of theOuter Moldavids (i.e., in the Tarcău Nappe) from the Covasna Valley(section C in Fig. 1), displaying the succession of its three members(lowest first):

(i) The Lower Member that is composed of 40 m laminated blackshales with rare intercalations of sandstones, the later yieldingparallel laminationand frequently ripplemarks. Commonbedsorlenses of siderites and limestones are present. Thin cm up to dmmudstones, wackestones and marls were mostly encountered inthemiddle part of this unit. Rare cmnodular and lenticular chertscould be also observed, mainly towards the upper part of thismember (Figs. 3 and 4). The calcareous nannoplankton assem-blages of this unit are of Early Hauterivian-Late Barremian in age,covering the NC4 and NC5 calcareous nannofossil zones (Fig. 2).

(ii) The Middle Member, 80 m thick, is lithologically similar to theLower Member, being composed of laminated black shales, butwith thicker andmore frequent levels of lithic sandstones, aswellas interbedded black cherts (Figs. 3 and 4). Rarely, sideritic andmicritic limestones, aswell as lenticular and nodular siderites arepresent (Fig. 5). Breccias with granodiorite fragments alsooccurred. Based on calcareous nannofloral analysis, the age ofthis unit is Late Barremian–Early Albian, an interval covered bythe NC6, NC7 and NC8 calcareous nannoplankton zones (Fig. 2).

(iii) The Upper Member is made up of 40 m thick black shales andintercalated grey-green quartzous sandstones; it has a distin-guishing feature that is the common presence of glauconite.The decimetre-thick sandstones are separated by centimetre-thick black shales (Figs. 3 and 4). The sandstones representaround 70–80% of the whole Upper Member. We identified inthis lithological unit calcareous nannofloras belonging to theNC8, NC9 and NC10 (lower part) zones, which indicate an Earlyto Late Albian age (Fig. 2).

Fig. 2. Lithostratigraphy and biostratigraphy based on calcareous nannoplankton of theblack shale andCORBunits of theMoldavidnappes (Eastern Carpathians).NK— calcareousnannofossil zones and subzones after Bralower et al. (1989); NC— calcareous nannofossilzones and subzones after Roth (1983); UC — calcareous nannofossil zones and subzonesafter Burnett (1998); FO — first occurrence.



4.2. CORBs

In the investigated Moldavid sections, the black shale units aredirectly overlaying by CORBs (Figs. 2 and 3). CORBs were previouslydescribed as Bota–Botiţa Formation in the Audia Nappe and Cârnu–Şiclău Formation in the Tarcău Nappe (Săndulescu et al., 1981), eventhough they display a similar lithology. In all the above-mentionedunits, CORB deposition began during the upper part of the Albian(Fig. 2), in the NC10 calcareous nannoplankton zone of Roth (1983),respectively in the UC0 Zone of Burnett (1998).

In the Audia Nappe, the Upper Member of the Audia Formation(composed of organic-rich black shales interbedded with subquart-

zous sandstones rich in glauconitic levels is conformably covered bythe CORB of the Bota–Botiţa Formation, 125 m in thickness (Fig. 5).The top of the CORB is unconformably covered by the massivesandstones of the Siriu Formation (Figs. 2 and 3). The base of thislithological unit contains, in the Covasna Valley, centimetre-thickradiolarites alternating with red and green or even grey siliceousshales (Figs. 3 and 6). Thin cm pyroclastic tuff levels are also present.The middle part of the CORBs from the Audia Nappe is dominated byvariegated red and green shales. Towards the upper part, silts,variegated shales, micritic limestones, as well as centimeter-thicksandstones, with parallel lamination, and fine-grained conglomeratesare present. The thickness and the grain size of the sandstones and

Fig. 3. Lithostratigraphy of theblack shale and CORBunits in theMoldavids (EasternCarpathians). Legend: 1—Conglomerates andbreccias; 2— Sandstones and greywackes; 3— Siltstones;4 — Organic-rich black shales; 5 — Variegated red and green shales; 6 — Cherts and radiolarites; 7— Pyroclastic tuffs; 8 — Siderites; 9 — Limestones (mudstones and wackestones); 10 —

Marls; 11—Nodules; 12— Pinch and swell structures; 13— Trace-fossil burrows; 14— Foraminifera bioclasts; 15— Indeterminable calcareous bioclasts; 16— Spicules of siliceous sponges;17 — Radiolarian bioclasts; d1 — location of sedimentological logs of Figs. 4 and 6. Grain size classes: c—clay; t—silt; s—sand; g—gravel.



conglomerates increased towards the top of CORBs. The wholesequence yielded a coarsening upward tendency. The age of theBota–Botiţa Formation is Late Albian–Coniacian pro parte (Fig. 2), aninterval covered by the UC0–UC10 calcareous nannoplankton zones ofBurnett (1998).

At the Tarcău Nappe, in the exposed section of the Covasna Valley(section C in Fig. 1), the CORB unit, namely the Cârnu–ŞiclăuFormation (65 m in thickness), is conformably overlaying the UpperMember of the Audia Formation and is unconformably covered by thecalcareous turbidites of the Horgazu Formation (Fig. 2). The base ofthe CORB is composed of red radiolarian rocks with different degree ofchertification, interbedded with thin grey and green shales (Fig. 5). Afew centimetre-thick tuff intercalations also occur (Figs. 3 and 6).Within the upper part of the Cârnu–Şiclău Formation, red shales aremore numerous, being interbedded with green and light grey ones.These variegated couplets are themain lithological feature of this unit.Towards the top, the CORBs grade into a grey and reddish muddylimestone succession, interbedded with decimetre-thick lithic andbioclastic sandstones, with parallel lamination and ripple marks. Theamount of siliciclastic rocks increased to the upper part of the CORBunit; cm up to dm conglomerates and breccias occur. Some breccialevels contain fragments of granodiorites (Figs. 3 and 6).

Previous studies (Neagu, 1970; Ion and Szasz, 1994) indicate thatplanktonic foraminifera are very rare in the Cârnu–Şiclău Formation,and were identified only near its top. On a contrary, the lower part ofthis unit is rich in agglutinated foraminifers and commonly containspyritized radiolarians (Neagu, 1968, 1990; Bubík, 2005). Thecalcareous nannoplankton of the Cârnu–Şiclău Formation belong tothe UC0–UC10 biozones of Burnett (1998), indicating that the age ofthese deposits is Late Albian–Early Coniacian.

5. Discussion

5.1. Sedimentologic and genetic significance

5.1.1. Black shalesThe laminated organic-rich black shales are the main lithological

component of the Upper Valanginian–Upper Albian Audia Formation.They also occur,within the Albian–Cenomanian boundary interval, as cmlevels interbedded in the CORB units of the Moldavids. These shales arecomposed of phyllosilicates, quartz and subordinately siliceous bioclasts(radiolarian and sponge spicules); they also include carbonate bioclastswith foraminifera, bivalve and echinids (Grigorescu, 1971; Papiu andAlexandrescu, 1976; Grasu et al., 1988). In the Eastern Carpathians, inthe northern outcrop area of the Audia Nappe (Bistriţa Valley), the mainconstituent of the black shales is illite (21–56%), followed in descendingorder by detritic quartz, chalcedony and opal (jointly amounting 18–24%), chlorite (2–8%), calcite anddolomite (making together 1–15%), andpyrite (0–2%), after Papiu and Alexandrescu (1976).

A high content of organic matter was identified in the laminatedblack shales. In the Audia Formation (Audia Nappe), the total organiccarbon (TOC) of the laminated black shales varies from 1.05% up to3.35% (Balteş et al., 1984; Lafargue et al., 1994). In the same formation,but at the outer tectonic position (in the Tarcău Nappe), the averageTOC of the Lower Member is 3.2%, but some layers contain up to 5.5%of TOC (Filipescu et al., 1966; Balteş et al., 1984). The TOC decreases inthe Middle and Upper members down to 1.8%, and 1.1%, respectively.

The bitumen analyses indicate average values for C=78.86,H=10.87, N=0.30 and O=9.97 (Grasu et al., 1988). The above-mentioned authors concluded that the organic matter is mainlysapropelic, kerogen type I and subordinately humic.

The dominance of the illite, detritic quartz clasts, chloritized micas,associated with siliceous and carbonate bioclasts in the dark greyshales suggests that these deposits are mainly clastic and weredeposited from hemipelagic suspensions into a deep sea abyssal plainenvironment.

The dark grey up to the black colour of the shales is due to the anoxicdepositional conditions that led to the conservation of the organicmatter and to the reduction of the iron oxides, allowing the formation ofthe pyrite and siderite during diagenesis. Itwas assumed that the anoxicpalaeoenvironment was linked to the palaeotopography of the narrow

Fig. 4. Logs of representative sequences of the black shale units (the Audia Formation) inthe Covasna section (Tarcău Nappe). For lithological legend and location of logs, see Fig. 3.Legend of the sedimentary internal structures: 1.Massive; 2. Normal grading; 3. Horizontalparallel lamination; 4. Cross lamination, asymmetrical current ripples. Grain size classes: c—clay; t—silt; sf—fine sand; sm—medium sand; sc—coarse sand; g—gravel.



basin of theMoldavids, in which the restricted circulation did not allowtheexchangebetween surfaceandbottomwaters, leading to thedensitystratification. Noteworthy, the quantity of bitumen type A extracted inchloroform is low, between 0.07% and 0.13% (Grasu et al., 1988); hence,the potential for hydrocarbon generation isweak for the LowerMemberof the black shale units, moderate for the Middle Member andinsignificant for the Upper Member (Balteş et al., 1984).

5.1.2. SideritesThe presence of this type of rocks is a lithological feature of the

Lower Member of the Audia Formation. The siderites are intercalatedin the laminated black shales as nodules or stratiform, irregular lentils,yielding a variable thickness, cm up dm.

The chemical analysis of the siderites from the Audia Formation(LowerMember) indicate the presence of SiO2 (6–20%), Al2O3 (2–5%),Fe2O3 (0.5–19%), FeO (5–38%), CaO (5–40%), MgO (1–2) and MnO(0.01–0.8%) (Savul et al., 1965; Papiu and Alexandrescu, 1976). Thesedata show that the siderites are not exclusively composed of FeCO3, butthey are also marly, due to their significant content of silica andaluminium. The presence of Fe2O3 is probably due to the oxic con-ditions generated during late diagenesis stage. The genesis of thesiderites in the Moldavid black shales units started during an earlydiagenesis stage. We may suppose a scenario as proposed by Berner(1985) and Tucker (1991), suggesting that siderites precipitatedduring a high carbonate activity and a low sulphide one, in a methanicenvironment.

5.1.3. Cherts and radiolaritesSiliceous rocks were mainly identified in the black shale units of the

Moldavids (frequently in the Lower and Middle members of the AudiaFormation, and rarely in the Upper Member of this unit). Overall, they

occur in the Valanginian–Albian interval. These rocks are also present inthe lower part of the CORB unit of the Moldavid nappes.

The centimetre-thick black siliceous shales and cherts of the LowerMember of the Audia Formation, yielding pinch and swell structures,contain sponge spicules, radiolarian tests, as well as microcrystallinesilica; the latter component amounts to 50–70% (Grigorescu, 1971;Grigorescu and Anastasiu, 1976). As it is expected, the siliceous rocksof the Middle Member of the Audia Formation yielded a high contentof SiO2 (90%) and a less amount of Al2O3 (4%), Fe2O3 (1.5%), MnO(0.05%) and MnO (0.5%), according to Filipescu et al. (1966).

Cherts and radiolarites are also present in the lower part of theMoldavid CORB units (in the Upper Albian–Turonian interval), formingcentimetre-thick couplets with variegated shales. They yield differentdegrees of chertification. Possibly, towards the upper part they wereintrabasinal reworked, as they show horizontal parallel laminatedstructures. They mainly contain SiO2, over 80% (Papiu et al., 1983), andless amounts of Al2O3 (between 12 and 20%), and in order of decreaseabundanceFe2O3 (between4% and6%), K2O (up to 3.5%),MgO (up to2%)and CaO (up to 1%) (Papiu et al., 1983).

The siliceous levels represent pelagic and hemipelagic accumula-tions of a deep-sea abyssal plain environment. The sponge spicules,commonly occurring within the Valanginian–Albian interval, may bereworked form the deep shelf area. The silica source could be, at leastfor the Cenomanian–Turonian interval, enhanced by volcanism andalteration of the pyroclastic tuffs. The clayey intercalations associatedwith the siliceous rocks of the CORB contain agglutinated foraminif-eral assemblages with “Rhizammina” sp. and few elements of“Krasheninnikov fauna”, such as Praecystammina globigerinaeformis,Recurvoidella insueta and Trochammina gyroidinaeformis. The presenceof the above-mentioned assemblages argues also for an abyssalpaleoenvironment (Bubík, 2005; Melinte-Dobrinescu et al., 2009).

Fig. 5. a: Upper Barremian deposits of the Audia Formation (Middle Member in the Covasna Valley, Tarcău Nappe); b: Upper Albian deposits of the Audia Formation (Upper Memberin the Bicaz Valley, Audia Nappe); c: Upper Albian black shales (Upper Member of the Audia Formation) overlain by the CORB of the Bota–Botiţa Formation (Bota Valley, AudiaNappe); d: Upper Albian CORB of the Cârnu–Şiclău Formation (Covasna Valley, Tarcău Nappe).



5.1.4. Siliciclastic rocksThe siliciclastic rocks are characteristic for the Upper Member of the

Audia Formation. Sporadically, these rocks are present in the Lower andMiddle members of the Audia Formation and towards the upper part ofthe CORB units (i.e., Bota–Botiţa and Cârnu–Şiclău formations).

The siliciclasts are massive and normally graded sandstones,locally with basal fine-grained conglomerates, followed by sandstoneswith parallel lamination and ripple marks and by silts and shales;overall, they show a fining up tendency. Complete sequences as abovedescribed were only rarely encountered. Often, some terms aremissing, such as the basal conglomerates or the sandstones; hence,sequences containing conglomerates directly overlain by silts andshales are present. The thickness of these sequences, dominated bymassive and normal graded sandstones, is variable, from a maximum1.5 m in the Upper Member of the Audia Formation to less than 20 cmin the other analyzed stratigraphic units.

The described sequences could be assigned to classical turbidites,most of them incomplete. We identified several coarse sequences,representing the R3–S2 divisions, as described by Lowe (1982) andcharacterizing the middle and deep water fans (Mutti and Ricci Lucci,1972; Walker, 1978). Other sequences represent the Ta-e terms ofBouma (1962), most of them incomplete.

From petrographical point of view, the sandstone sequences showdifferences in the studied units. For instance, the sandstones of theLower and Middle members of the Audia Formation are lithic andcontain green anchimetamorphic clasts, probably derived from anouter source that is the East European Platform.

The arenites of theUpperMember of theAudia Formation aremainlyrepresented by sublithic or subquartzous sandstones, with high quartzcontent (up to 90%), followed by feldpars, micas, metamorphic andsedimentary lithoclasts, heavy minerals, as well as siliceous and car-bonate bioclasts (Grigorescu, 1971; Grigorescu and Anastasiu, 1976).Some levels contain authigenic glauconite, up to 14%. The source of theUpper Member arenites is also an outer (eastern) one (Săndulescu,1984).

In the CORB units of the Moldavids nappes (i.e., Cârnu–Şiclău andBota–Botiţa formations), the sandstones, which occur towards theupper part (mainly in the Coniacian stage) are lithic, containing meta-morphic and sedimentary rock fragments, muscovite and a carbonatecement. The provenance of the clasts depends on the location in thebasin. In the internal area of the Moldavid basin, the fragments derivedfrom an inner, western, source (Papiu and Alexandrescu, 1976). In theOuter Moldavid basin, the origin of the clasts is multiple, being locatedin the intrabasinal flexural bulges as well as in an outer, eastern, sourcethat is the Eastern European Platform (Vârban, 2003).

5.1.5. Limestones and marlsIn the LowerMember of theAudia Formationexposed in theCovasna

Valley, carbonate levels occur as centimetre marls or biomicrites (withb10% bioclasts), containingmainly planktonic foraminifera (Grigorescu,1971). In the Middle Member of the above-mentioned unit, thecarbonate levels are marls, as well as bioaccumulated limestones withterrigenous material and metasomatic calcite cement. The analyses ofthese carbonate levels from the northern part of the Audia Nappeindicate the following composition (Papiu and Alexandrescu, 1976):SiO2 (6–70%), Al2O3 (0.5–8.5%), Fe2O3 (0.3–12.5%), FeO (3.5–38%), CaO(5.5–44.5%), MgO (1.5–9%) and K2O (0.3–1.5%). The mineralogicalcomposition of the same rock is illite (0–21%) chlorite (0.5–5%), quartzand chalcedony (0–60%), feldspar (0–8.5%), calcite and dolomite (0–84%), as well as pyrite (0–1.30%). The upper part of CORBs from theMoldavid external structures (i.e., the Tarcău and the Vrancea nappes)

Fig. 6. Logs of representative sequences of the CORB unit (the Cârnu–Şiclău Formation)in the Covasna section (Tarcău Nappe). For lithological legend and location of logs, seeFig. 3. Grain size classes: c—clay; t—silt; sf—fine sand; sm—medium sand; sc—coarsesand; g—gravel.



becomemore carbonatic, having a high content of CaCO3 (36–44%) andless of Al2O3 (3.8–8.3%) (Grasu et al., 1988). Probably, these marly andcarbonate rocks are hemipelagic accumulations, with clayminerals andbiogenic material (sponge spicules, echinoids and foraminifers) thatderived from the shelf.

5.1.6. Variegated couplets of CORBThe variegated (red, green and light grey) centimetre-thick couplets

represent the main part of investigated CORBs (i.e., Bota–Botiţa andCârnu–Şiclău formations), except for the lowest part of these units(mainly composed of red shales, radiolarites and cherts), and theuppermost part, where frequently siliciclastic rocks occur. The redshales are mainly composed of clay minerals, detritic quartz andbioclasts with radiolarians, foraminifera and microcrystalline silica. Thegreen terms of variegated couplets are composed of phyllosilicates inthe Audia and Tarcăunappes (Papiu et al., 1983)while, in the outermostMoldavids structures (i.e., the Vrancea Nappe) they are mainly marls orlimestones (mudstones or wackestones) (Vârban, 2003).

In the Audia and Tarcău nappes, the red shales contain illite (35–51%),quartz and chalcedony (21–43%), kaolinite (5–10%), feldspars (4–6%);chlorite (∼3%); hematite (2–3%), as well as calcite and dolomite (0.2–1.5%). The chemical analysis indicate a high amount of SiO2 (60–70%),togetherwithAl2O3 (12–19%), Fe2O3 (4–6%), FeO (∼1%),MgO (∼2%), CaO(∼1%), Na2O (0.6–0.7%), K2O (2–3%) and TiO2 (∼0.5%). The greenshales yielded illite (41–44%), quartz and chalcedony (29–31%), kaolinite(8–10%), feldspars (∼6%), chlorite (5–8%), hematite (0.4–0.7%), calciteand dolomite (1–2%). The green shales contain, similar to the red ones,high percentages of SiO2 (63–64%), and, in descending order, Al2O3

(∼17%), Fe2O3 (2–3%), FeO (1.7–2.9%), MgO (∼2%), CaO (∼1%), Na2O(0.6–0.7%), K2O (2–3%) and TiO2 (∼0.5%) (Papiu et al., 1983).

In theAudia andTarcăunappes, thehematite of the red levels is up to3%, while in the green levels the hematite is less than 0.5%. Both red andgreen levels contain terrigenous clay minerals, such as illite, kaoliniteand chlorite. After Papiu et al. (1983), some chlorites, at least thoseoccurring towards the lower part of CORB units, are produced by thediagenetic weathering of the pyroclastic tuffs. However, the pyroclas-tites are present only in the older parts of the CORB units (within theUpper Albian–Cenomanian interval), while variegated couplets aremainly present towards the upper part, within the Turonian–Coniacianinterval.

Vârban (2003) suggested that, at least for the CORBs of theMoldavidouter structures (i.e., Vrancea Nappe), a primary depositional alterna-tion of red shales and white limestones (mudstones and wackestones)could be imagined. The above-mentioned author hypothesized thatthe deposition of these couplets took place exclusively under oxicconditions; as argues, he invoked the absence of any black shales andpyrite and the presence of bioturbations both in the red and green termsof the couplets. The iron oxide source is represented by continentallateritic soils. During diagenesis, reducing interstitial fluids migratedpreferentially in the more permeable carbonate levels overprinting thegreen colour.

During redox processes, the oxygenation should be produced byoxygen-rich currents, which induced to the clastic material the redcolour by hematite formation. It is also possible that an arid climate ofthose times led to the accumulation of red soils on emerged coastalplains; transgression could have led to redeposition of sediments rich inFe-hydroxides into themarine environment, generating the CORBs thatare now preserved. We assume that the chlorites were oxidized on theland at different intervals, related to short regional climate fluctuations,resulting in the alternating red and green shales. A similar lithologyconsisting of variegated red and green coupletsmay be produced by theintermittent occurrence of bottom-oxygenated currents.

5.1.7. Pyroclastic tuffsPyroclastic tuffs were identified within the lower part of CORB units

(in the Bota–Botiţa and Cârnu–Şiclău formations of the Audia and,

respectively, Tarcău nappes). They occur as green-yellowish centi-metre-thick strata, showing a good lateral continuity. The chemicalanalysis indicates the following content (Papiu and Alexandrescu,1976): SiO2 (75–91%), Al2O3 (3.5–8.5%), Fe2O3 (0.5–2.5%), FeO (0.7–1%),CaO (0.1–0.4%), MgO (0.5–6%), K2O (0.5–1.5%), Na2O (0.1–0.5), TiO2

(0.1–0.4) and P2O5(0.02–0.7). The same authors identified in themineralogical composition: quartz and chalcedony (jointly amounting66–87%), feldspars (1.8–5.25), illite (8.5–19%) and chlorite (2.20–8.2%).Sporadically, pyrite also occurs.

Some authors assumed that the sources of these rocks are the calc-alkaline magmatic eruptions of an island arc type (Rădulescu andDimitrescu, 1982). This volcanismwasassociatedwith thedeformationsthat took place at the end of the Lower Cretaceous in the WesternTransilvanids (Săndulescu, 1984), located now in the Apuseni Moun-tains. The illite, kaolinite and the terrigenous clasts were transportedfrom the continent. After accumulation, the pyroclastites were involvedin diagentical processes. The resulted silica favoured the development ofthe radiolarian communities (Papiu and Alexandrescu, 1976). The argil-lization processes continued during middle and late diagenesis stages.

5.2. Palaeogeographic setting

The Moldavids, where the black shales accumulated during EarlyCretaceous times, constitute one of the largest nappe systems of theEastern Carpathians. The black shale deposition took place in a verydeep environment, on the abyssal plain of theMoldavian Trough, withdepths near the calcite compensation depth (CCD). The occurrence ofthe black shales in the Eastern Carpathians is linked to the existence ofa narrow basin, formed on a thinned continental crust of the passivenorthern margin of western Tethys Realm (Săndulescu et al., 1981;Papiu et al., 1983). There, the anoxic to dysoxic bottom waters led tothe deposition of organic-rich black shales that accumulated at a lowrate, from 2 up to 5 cm/kyr (Ştefănescu and Melinte, 1996).

A similar Early Cretaceous tectonic setting as recorded in theEastern Carpathians characterizes the Silesian Nappe of the OuterPolish Carpathians. This represents a deep, long trough, restrictedtowards N by the Subsilesian Submerged Ridge and towards S by theSilesian Ridge (Książkiewicz, 1975; Golonka et al., 2000). In thistrough, the Lower Cretaceous sediments were also deposited aroundthe CCD (Golonka et al., 2002).

Within the Albian–Cenomanian boundary interval, due to theoverthrusted processes, which affected the Outer and Marginal Dacids,situated at the western part of the Moldavid Basin, and to theappearance offlexural bulge zones (Bădescu, 2005), previously ascribedto intrabasinal cordilleras (Murgeanu, 1937), the circulation patternchanged in the Moldavian Trough. This modification is indicated by thecommon presence of macroscopically visible authigenic glauconite inthe Albian sandstones of the Upper Member of the Audia Formation,linked to the circulation, in the Moldavid Basin, of the oxygenatedturbidity currents. Towards the end of the Albian, the anoxic–dysoxicregime (expressed by the deposition of the dark grey laminated shalesinterbedded with glauconite sandstones) shifted to an oxic one (whichled to theoccurrenceof CORBs). In allMoldavidunits, towards the endofthe Albian stage, black shaleswere progressively replaced by red shales.

In other European Tethyan regions, such as the central Italy, CORBsedimentation started from in the Late Albian–Early Cenomanianinterval (Hu et al., 2006; Hu et al., 2009). In the Pieniny Klippen Belt ofthe Polish Inner Carpathians (Bąk, 1998), as well as in the Rhenodanu-bian Flysch Zone of Austria (Wagreich et al., 2006) the beginning of theCORB sedimentation was in the Late Albian. A comparable setting of theCORB deposition as recorded in the outer part of the Eastern Carpathianswas identified in the Polish Outer Carpathians, where Upper Cretaceousred shales also directly overly the black shales (Bąk, 2006, 2007).However, the duration of theCORB sedimentation is different in the sub-basins of the Polish Outer Carpathians, where it was terminated by theinflux of terrigenous turbidites (Geroch et al., 1967), similarly to the



Austrian Rhenodanubian Flysch (Wagreich et al., 2006). We maysuppose that the beginning and the end of the CORBs is not a globalevent, but it depends of different palaeogeographic and palaeoenviron-mental settings, beingalsodrivingby the regional tectonics and thebasinconfiguration.

6. Conclusions

In the Eastern Carpathians, black shales intercalatedwith carbonates(marls and micrites) firstly appeared in the Valanginian, with the latterrocks progressively diminishing up to the Aptian. During the LateValanginian–Late Barremian interval, carbonate hemipelagites andpelagites, with rare siliceous rocks, accumulated in the Moldavids.These sediments of an abyssal plain are interbedded with thin distalsiliciclastic and bioclastic turbidites. Taking into account the presence ofthe green anchimetamorphic clasts of the turbidites, we may assume aprovenance from the East European Platform.

For the Late Barremian–Early Albian, a similar palaeoenvironmentalsetting could be assumed, consisting of anoxic sedimentation in theabyssal plain,with rare turbidites having the sameeastern, outer, sourcethat is the East European Platform. Additionally, the occurrence ofsiliceous deposits in the above-mentioned interval is possibly linked tothe shift of CCD depth, or the increasing subsidence, together with thechange in the bottom current circulation.

During the Early Albian–Late Albian interval, the pelagic sedimen-tation shifted to a turbidite one. The anoxic–dysoxic environment isreplaced by a more oxic one, a change that is indicated by the richauthigenic glauconite levels encountered in sandstones, implying thepresence of Fe3+.

A major modification in the depositional regime occurred withinthe latest Albian, when red, non-calcareous shales, were depositedabove the black shales interbedded with glauconitic sandstones. Thedeposition of the Upper Cretaceous red, mostly non-calcareous shalesis prevalent in all tectonic units of Moldavids and signifies a majorchange from an anoxic or dysoxic environment to an oxic depositionalenvironment.

From the end of the Albian up to the Cenomanian, CORBs weredeposited as pelagic and hemipelagic sediments in the abyssal plain,where red and green radiolarites, siliceous muds and thin pyroclas-tites accumulated during fall out processes. Notably, dark grey shalesare still present in the lower part of the Moldavid CORB units. Duringthe upper part of the CORBs (the Turonian–Coniacian interval), adecrease of the basin depth could be assumed. The pelagites and thehemipelagites of the abyssal plain were progressively replaced by thesiliciclastic deposits of turbidite lobes. This change is also supportedby the fluctuation of foraminiferal assemblages. Within the lower partof the CORB (i.e., Upper Albian–Upper Cenomanian), agglutinatedforaminiferal assemblages with “Rhizammina” sp. and few elements of“Krasheninnikov fauna” (such as Praecystammina globigerinaeformis,Recurvoidella insueta and Trochammina gyroidinaeformis) are indica-tive of an abyssal paleoenvironment. Upwards in the CORB, within theUpper Cenomanian–Lower Turonian interval, as the palaeoenviron-ment shifted from the abyssal to the lower slope, the microfaunalassemblages become dominated by the Recurvoides–Karrerulina–Uvigerinammina. In the Upper Turonian–Coniacian CORB, the agglu-tinated foraminiferal assemblages are dominated by Nothia sp., whichoccur together with Recurvoides and Karrerulina taxa, suggesting alower slope palaeoenvironment.

To summarize, the identified lithofacies types suggest the presenceof two major coarsening upward sequences: (i) A Valanginian–UpperAlbian sequence, which contains in the lower part sediments depositedin an abyssal plain, such as dark grey shales, siderites, cherts, marls andlimestones, with thin turbidites interbedded; the upper part is mainlycharacterized by the presence of coarser turbidites, related to a middlefan palaeoenvironment; (ii) An Upper Albian–Coniacian sequence,which contains in its lower part deposits of an abyssal plain, such as

radiolarites, as well as red and green shales, alternating with thinturbidites; the upper part mainly includes turbidites of an outer deepwater fan.

The presence of the above-mentioned sequences indicates thatmajor progradations took place in the Upper Albian and in the LowerConiacian intervals. These processes are related to the increase of thesediment supply during the two above-mentioned intervals, or to thedecrease of the basin depth, by the migration of the depocentre. Thesechanges could be linked to the tectonic activity of those times,especially to the Intra-Albian tectonic phase that was very intense inthe Eastern Carpathian area, including the Moldavids.

Probably many factors forced the worldwide change of the EarlyCretaceous anoxic conditions (represented by black shales) to the LateCretaceous oxic conditions, when CORBwhere deposited in the Tethys(Jansa and Hu, 2009; Wagreich, 2009). The most common assump-tions suggested changes in the ocean circulation, ocean chemistry,bioproductivity, and climate. Study of the termination of the CORBdeposition in the Carpathian area placed in a foreground the influenceof regional factors, such as tectonics and regional palaeogeographicsetting. This conclusion emanates from our study demonstrating thatthe CORB deposition began in the Eastern Carpathians during the LateAlbian only in the outer units (i.e., the Moldavids), effected regionallyby the intensification of mid-Cretaceous tectonic movements.

Acknowledgments

We thank to Dan Jipa (National Institute ofMarine Geology andGeo-ecology, GEOECOMAR Bucharest) for the fruitful discussion andcomments on an earlier version of this paper. The authors are alsoindebted to Titus Brustur and Stefan-Andrei Szobotka (National Instituteof Marine Geology and Geo-ecology, Bucharest), for the assistanceduring field trips in the Eastern Carpathians. This paper is a contributionto the IGCP Project 555 ‘Rapid Environmental/Climate Change in theCretaceous Greenhouse World: Ocean–Land Interaction’. We arethankful to Xiumian Hu for his helpful comments. The authors areindebted to Luba Jansa and Robert Scott, who made useful suggestionsand improvements. We are indebted to Michael Wagreich, whomade avery useful and careful review that significantly improved this paper.Wethank the financial support of the Grant CNCSIS - Programme typeEUROCORES ESF no.2/2009 (integrating Project FROMSOURCE TOSINK-A TOPO-EUROPE Collaborative Project).

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